As the power consumption of computing systems has become a matter of concern, most present day systems include sophisticated power management functions. A common framework is to define both “performance” states and “power” states for a computer system's processor (which may include multiple processing cores). A processor's performance is its ability to do work over a set time period. The higher a processor's performance the more work it can do over the set time period. As such, a processor's power consumption increases as its performance increases.
A processor's performance can be adjusted during runtime by changing its internal clock speeds and voltage levels. Thus, a processor's different performance states correspond to different clock settings and internal voltage settings so as to effect a different performance vs. power consumption tradeoff. According to the Advanced Configuration and Power Interface (ACPI) standard the different performance states are labeled with different “P numbers”: P0, P1, P2 . . . P_R, where, P0 represents the highest performance and power consumption state and P_R represents the lowest level of power consumption that a processor is able to perform work at. The term “R” in “P_R” represents the fact that different processors may be configured to have different numbers of performance states.
In contrast to performance states, power states are largely directed to defining different “sleep modes” of a processor. According to the ACPI standard, the C0 state is the only power state at which the processor can do work. As such, for the processor to enter any of the performance states (P0 through P_R), the processor must be in the C0 power state. When no work is to be done and the processor is to be put to sleep, the processor can be put into any of a number of different power states C1, C2 . . . C_S where each power state represents a different level of sleep and, correspondingly, a different amount of time needed to transition back to the operable C0 power state. Here, a different level of sleep means different power savings while the processor is sleeping.
A deeper level of sleep therefore corresponds to slower internal clock frequencies and/or lower internal supply voltages and/or more blocks of logic that receive a slower clock frequency and/or a lower supply voltage. Increasing C number corresponds to a deeper level of sleep. Therefore, for instance, a processor in the C2 power state might have lower internal supply voltages and more blocks of logic that are turned off than a processor in the C1 state. Because deeper power states corresponds to greater frequency and/or voltage swings and/or greater numbers of logic blocks that need to be turned on to return to the C0 state, deeper power states also take longer amounts of time to return to the C0 state.